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298                         FUELS (GENERAL METHODS)

of the proper mesh, that remaining on the sieve must be again powdered
and sieved until the whole has passed through.

GENERAL METHODS
1. Chemical Analysis

This usually includes determinations of the moisture, ash, coke and
volatile substances, and sulphur (see r, 2, 3 and 4). Of interest in some
cases are determinations of the phosphorus, carbon and hydrogen, nitrogen
and oxygen (see 5, 6, 7 and 8).

1.  Determination of the Moisture.—About 5 grams of the substance,
not too finely powdered (say, to pass through a sieve of 250 meshes per
sq. cm.), are dried in an oven at 105-110° to constant weight, the sample
being placed in a covered platinum dish or crucible or between two watch-
glasses ; as a rule the drying does not require more than two hours.    Since
dry coal dust, especially that of highly bituminous coal and lignites, tends
to oxidise in the air, any increase in weight should be neglected and the
preceding weight taken as constant.    In such cases, when highly exact
determinations are required, the drying should be carried out in a boat
in a current of carbon dioxide.

With washed coal, peat and certain earthy lignites, the determination
of the hygroscopic moisture is preceded by that of the water oj imbibition.
For this purpose, a large quantity (at least I kilo) of the coal, coarsely
ground and weighed, is left to dry in the air, the diminution in weight repre-
senting the water of imbibition. The substance thus obtained is powdered
and used for determining the hygroscopic moisture and other constituents.

2.  Determination of the Ash.—From 2 to 5 grams of substance (that
used for the determination of moisture will serve) are incinerated either
in a platinum dish in a muffle or in an open, inclined platinum crucible
resting on a perforated asbestos card over a bunsen flame, care being taken
to heat gently at first to drive off the volatile substances and to increase
the temperature gradually to redness.

In some cases the ash is analysed chemically to determine its principal
components and its alkalinity; it may also be examined from the point
of view of its fusibility.

3.  Determination   of   the  Coke   and   Volatile   Substances.—i
gram of the substance (coals rich in volatile matters are best coarsely pow-
dered, say to pass through a sieve of 100 meshes per sq. cm.) is weighed
in a platinum crucible 30-35 mm. high, which is placed covered on a triangle
of platinum wire arranged so that the bottom of the crucible is 3 cm. above
the apex of a bunsen burner giving a flame 18-20 cm. high.    When the
burner is lighted, it is placed at once under the crucible and maintained
as long as luminous flames issue from the edges of the crucible;   when
these cease—usually after not more than two minutes—the flame is extin-
guished and the crucible, without opening it, placed in a desiccator, allowed
to cool, and weighed.   The residue in the crucible, less the ash, is regarded
as coke (fixed carbon), and the loss in weight, less the moisture, as thepper by means of hydrogen sulphide, filtering, eliminating